Mesoporous Silica with Different Channel Widths for Cs⁺ Adsorption: A Molecular Dynamics Simulation Study

Mesoporous silica materials have important application prospects in the field of radioactive Cs⁺ adsorption and separation due to their ordered channel structures and high specific surface area. Understanding the microscopic adsorption and diffusion mechanism of Cs⁺ in mesoporous silica channels is key to designing efficient adsorbents. In this study; molecular dynamics simulations were employed to construct mesoporous silica models with channel widths of 2 nm; 4 nm; and 6 nm. The density distribution; radial distribution function (RDF); hydrogen-bonding network; and mean square displacement (MSD) of Cs⁺, NO₃^-, and H₂O in the channels were systematically investigated. The results show that Cs⁺ mainly adsorbs near the channel interface. When the channel width is 2 nm; Cs⁺ is completely captured at the interface; while as the channel size increases; Cs⁺ gradually appears in the center of the channel; indicating weakened adsorption capacity. RDF analysis reveals that approximately 60% of Cs⁺ are within 0.25 nm of the surface -OH groups; forming inner-sphere complexes; while the rest are outer-sphere complexes. The RDF peak of NO₃^- appears at 0.4 nm; confirming its indirect adsorption via electrostatic interaction with Cs⁺. Hydrogen-bonding network analysis shows that the average number of hydrogen bonds per water molecule at equilibrium is about 3.45; which slightly decreases with increasing channel size. MSD results indicate that Cs⁺ has the weakest diffusion capacity; which increases with channel width. This study reveals the regulation mechanism of channel size on Cs⁺ adsorption and diffusion at the atomic level; providing theoretical guidance for the channel engineering design of mesoporous silica-based adsorbents.